1. Field of the Invention
The present invention relates to an optical element in which the viewing angle can be changed, an optical device, a mobile information terminal, and a manufacturing method for a microlouver that is used as an optical element.
2. Description of the Related Art
Liquid crystal display apparatuses that can be viewed from a wide angle have come into practical use in conjunction with technological advancement in recent years. Also, mobile information terminals having a liquid crystal display apparatus continue to become more pervasive. In such mobile information terminals, a wide viewing angle is desirable when acquired information is to be shared with and viewed by others. However, in mobile information terminal applications, there may be cases in which the screen in is not meant to be viewed by others. It is therefore desirable to have the capability of selecting a wide viewing angle or a narrow viewing angle in accordance with service conditions.
An LCD that satisfies the above-described needs is a microlouver (micro-lens sheet) in the form of a film that is laminated to the display screen as described in Japanese Laid-open Patent Application No. 10-197844 (paragraph 0035) and Convertech (vol. 32, no. 6, pp. 6 to 7, 2004). Japanese Laid-open Patent Application No. 10-197844 describes an approach whereby a microlouver that can widen the viewing angle is mounted on the display screen only when a wide viewing angle is used, and the micro-lens sheet is removed when a narrow viewing angle is used.
A microlouver 32 is laminated onto the display panel 29, as shown in FIG. 1. A conventional microlouver 32 has light-absorbing and transparent layers alternately disposed at regular intervals in the film surface. In the particular case that the light-absorbing layers extend in the vertical direction of the screen, the light emission in the horizontal direction of the screen is reduced. For this reason, persons positioned in the horizontal direction of the screen cannot view information on the screen.
However, the display apparatus described in Japanese Laid-open Patent Application No. 10-197844 has the following problems. As described above, the microlouver has an alternating structure of light-absorbing and transparent layers at a constant period on the surface of the film. Conversely, when a thin display having a constant pixel pitch is used as the display apparatus, a phenomenon referred to as “Moire fringes” occurs due to interference between the constant period of the microlouver and the pixel pitch of the thin display.
This phenomenon is described with reference to FIG. 2. The microlouver has a spatial arrangement such as that shown in the lower left portion of FIG. 2 because the microlouver has light-absorbing layers 30 and transparent layers 31 disposed at a constant period. The center lower portion of FIG. 2 describes the spatial arrangement in terms of spatial frequency. In particular, since the microlouver has a repeating period in a single direction, a regular peak arrangement is observed within the spatial frequency shown in the center lower portion of FIG. 2. The peak arrangement coordinates correspond to the positions of the integral multiples of the vector Pl. In addition, the magnitude of vector Pl is equal to the reciprocal of the microlouver period L.
Examples of a thin display include a liquid crystal display apparatus, an organic EL display, and a plasma display. These displays are configured with pixels 33 arranged at regular spatial intervals. For this reason, the displays will have regular two-dimensional peaks in terms of spatial frequency, as shown in the center upper portion of FIG. 2. The coordinates of these peaks are expressed by the position n·Px+m·Py of the integral multiple of the vectors Px and Py.
The right-hand portion of FIG. 2 shows the result of superimposing the microlouver and the thin display in the manner described above. The position of each peak is equal to the integral multiple l·Pl+n·Px+m·Py of Pl, Px, and Py. Among the combinations of the three integers, the integer having the shortest length of l·Pl+n·Px+m·Py has the largest period. For example, it is apparent that P1-Px in the right-hand portion of FIG. 2 is the shortest distance from the origin and that the period is a long period.
As described above, a long periodic structure is produced by superimposing the microlouver and the pixel pitch. This is known as a moire stripe.
The above-described condition occurs in the same manner in the liquid crystal display apparatus of Japanese Laid-open Patent Application No. 10-197844. When the PDLC layer is in a scattering state, interference between the microlouver period and the pixel pitch of the display screen is masked by the scattering of the PDLC layer and Moire fringes cannot therefore be noticed. However, when the PDLC layer is in a transparent state, interference between the two is generated, and Moire fringes can be noticed.
Moire fringes such as those described above are not limited to microlouvers, but are a phenomenon that is widely observed between periodic structures. Widely used in a backlight for a liquid crystal display apparatus, for example, is a structure in which a prism sheet is used for improving front surface brightness. The prism sheet has regular prism rows. For this reason, similar Moire fringes are produced when the prism rows and liquid crystal display panel are superimposed. A diffusion plate (diffuser) is ordinarily inserted between the prism sheet and the liquid crystal display panel in order to avoid such a situation. It is for this reason that the effect of improving the front surface brightness is typically diminished by the diffusion plate.
The problems described below also occur in a liquid crystal projector. The liquid crystal light bulb that forms liquid crystal projector images normally has a regular pitch. Display light projected by the liquid crystal bulb is projected onto a screen. In such a case, the surface of the screen is given a geometric structure to control the diffusion direction of the display light. When the geometric structure takes a periodic arrangement, Moire fringes are generated at the pixel pitch of the liquid crystal light bulb.
As described above, Moire fringes are a widely problematic phenomenon in display information bodies having a fixed pixel pitch.
In view of the above, Japanese Laid-open Patent Application No. 2001-51125 (paragraph 0009) discloses a backlight for a liquid crystal display apparatus in which convex or concave dots are disposed in parallel or in series while kept parallel to the light-incident end face portion on the front or back surface of the light guide plate. The serial or parallel intervals are obtained by a procedure in which the distance from the light-incident end face portion to the first serially or parallelly disposed dots is divided by the square root of 1.5 to 1.8, and by dividing the immediately preceding interval distance by the square root of 1.5 to 1.8. Luminescent lines can thereby be prevented.
Japanese Laid-open Patent Application No. 2004-252329 (abstract) discloses a reflective plate for a reflective liquid crystal display apparatus in which a concavo-convex surface comprising a plurality of concavities is formed on the surface of the reflective layer, and groups of concavities and convexities are randomly disposed in order to prevent the occurrence of Moire fringes.
Japanese Laid-open Patent Application No. 2005-38746 (paragraphs 0007 and 0008) discloses a backlight in which an excident-light control plate is disposed between the light guide body and the liquid crystal panel, and a plurality of convexities are disposed over the entire excident-light control plate at random intervals so that the occurrence of Moire fringes is prevented when light from a light source is incident to the side surface of a light guide body, and planar light from the excident plane of the surface of the light guide body is incident to the liquid crystal panel disposed above the light guide body.
The inventions disclosed in Japanese Laid-open Patent Application Nos. 2001-51125, 2004-252329, and 2005-38746 prevent Moire fringes, but they represent techniques for preventing Moire fringes produced by light incident from the backlight to the liquid crystal panel, or by light incident from the reflective layer of the reflective liquid crystal display apparatus to the liquid crystal layer. The prior art does not prevent Moire fringes generated by providing microlouvers that vary the viewing angle, as in the present invention. The objects of the present invention cannot be solved when such prior art is applied to a microlouver.